The present invention relates to a free-piston engine having a fluid pressure unit, comprising a cylinder and a piston arranged within the cylinder and limiting one side of a combustion room, said piston reciprocating within the cylinder between a first position in which the volume of the combustion room in the cylinder is at a maximum and a second position in which the volume of the combustion room in the cylinder is at a minimum, the piston including a plunger-shaped piston extension cooperating with the fluid pressure unit, said fluid pressure unit being equipped with an auxiliary means for displacing the piston in a direction to its first position, said auxiliary means comprising a connecting channel adapted to communicate a liquid pressure source with a room in which the plunger-shaped piston extension extends and which room is closed on one side by an axial face of the piston extension which is directed such that, by pressure loading this axial face, the piston is moved towards the first position.
In a known embodiment of such a free-piston engine having a fluid pressure unit (see EP-A-0 254 353), said axial face is formed on the plunger-shaped piston extension and the connecting channel of the room of the auxiliary means is connected to a low pressure container during normal operation. In a situation in which the auxiliary means should be used for bringing the piston to its bottom dead centre, such as with a so-called "misfiring", a two-way valve in the connecting channel is switched such that the room is communicated through the connecting channel with a source of higher pressure, such as a compression pressure accumulator.
This prior art free-piston engine has the disadvantage that during normal operation with each compression and expansion stroke of the piston, the reduction and enlargement of the volume of the room cause hydraulic liquid to be forced out from and sucked-in again into said room. This leads to great displacements of hydraulic liquid adversely affecting the efficiency of the free-piston engine. Also, at the start of the expansion stroke of the vehicle, where the piston acceleration is very great, cavitation is caused in the hydraulic liquid sucked-in into said room because this hydraulic liquid cannot follow the acceleration of the axial face formed on the piston. This cavitation is highly undesirable because of the detrimental effects caused thereby, such as vibrations and erosion.
It is an object of the present invention to provide a free-piston engine having a fluid pressure unit, wherein said problem is removed in a simple but effective manner.